Cross-linked cellulose nano-sponges: a small angle neutron scattering (SANS) study
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Cellulose nano-sponges (CNS), obtained by cross-linking TEMPO oxidized and ultra-sonicated cellulose nano-fibers (TOUS-CNFs) with branched polyethyleneimine (bPEI), underwent here a systematic small angle neutron scattering investigation, by varying the amount of cross-linker and the water content. The aim was to provide experimental evidence of nano-porosity in the TOUS-CNF network of these nano-sponges (CNSs) by investigating the water nano-confinement geometries in the adsorbent material. Moreover, we also verified how the breaking/reformation of specific intermolecular hydrogen bond interactions between water and the chemical groups present in the architecture of the CNSs could contribute to regulate the water adsorption process observed at macroscopic level. The analysis of the experimental data, performed in terms of the correlation length model, allowed us to extract the short-range correlation length ξ, interpreted as a very first indirect estimation of the effective nano-dimension of the cavities produced by the cross-linking of the reticulated cellulose nano-fibers. From the model, power-law (n) and Lorentzian (m) exponents have been also obtained, associated with the density of TOUS-CNFs at high (larger than hundreds of Å) and low (~ 10–100 Å) spatial scales, respectively. These parameters were all sensitive to the structural variations induced by the progressive uptake of water on the bPEI/TOUS-CNF sponges with different bPEI:TOUS-CNF (w/w) ratios. Finally, we investigated the effect of the addition of citric acid in the CNS formulation, confirming its role in increasing cross-linking density and sponge rigidity. The obtained results appear crucial in order to rationalize the design of these sponges and to track the changes in the ability of the final products as efficient nano-confinement systems for water.
KeywordsCellulose nano-fibers TEMPO oxidation SANS technique Nano-porous materials Cellulose nano-sponges
The authors acknowledge the CERIC-ERIC Consortium for the access to experimental facilities and financial support. The project was funded by Regione Toscana, NanoBonD (Nanomaterials for Remediation of Environmental Matrices associated to Dewatering, Nanomateriali per la Bonifica associata a Dewatering di matrici ambientali) POR CReO FESR Toscana 2014–2020-30/07/2014-LA 1.1.5 CUP 3389.30072014.067000007. This work benefited from the use of the SasView application, originally developed under NSF award DMR-0520547. SasView contains code developed with funding from the European Union’s Horizon 2020 research and innovation program under the SINE2020 project, Grant Agreement No 654000.
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